On the surface tension of molten salts and its temperature dependence

A new equation, based on Eyring statistical thermodynamic theory, has been developed to calculate the surface tension and its temperature dependence of high-temperature molten salts. The derived equation has been applied to the calculation of the temperature dependence of the surface tension of molten alkali halides. The model used here is analytical and free of interatomic potentials usually used in computer simulation. The only data needed for the calculation are the sublimation energy and the temperature dependence of the liquid density, which are readily available. The need for such a model stems from the fact that the data in the literature are scarce and the temperature range studied is limited by the experimental demand of high temperatures and clean surface environment, which is difficult to attain. The ratio of surface to bulk Madelung constant was calculated to be 0.95, which is found to be constant for all molten salts studied. This finding is interesting and is presented for the first time. The error in this estimate did not exceed 1.3%. The values calculated do commensurate with the available experimental values for many of the molten salts. The results of temperature-dependent surface tension calculations of molten LiCl (910–1150 K), NaCl (1080–1230 K) and KCl (1050–1300 K) are fitted as = 223.5 − 0.0814 (T-Tm), = 114.4 − 0.091 (T-Tm) and = 91 − 0.066 (T-Tm) (mJ m−2), respectively, where the surface tension decreases linearly with temperature and agrees well with the existing data.